Degradation: Collagenases, Bacterial, and Mammalian

Because of its triple helical structure stabilized by hydrogen bonds, the collagen molecules are quite resistant to enzymatic degradation in their native configuration. They can be degraded by collagenases. The first of these to be isolated were of bacterial origin, specifically, Clostridium Histolyticum. These enzymes are quite specific for collagen but will also degrade gelatin, which is denatured collagen.

They are inhibited by cysteine and other SH-compounds and by EDTA, a chelator for divalent cations. These enzymes are specific for peptide bonds involving glycine in a collagen helix conformation. Because of the abundance of this amino acid in collagen (every third residue) this enzyme generates a large number of small peptides.

The first enzyme derived from animal tissue capable of degrading collagen at neutral pH was isolated from the culture fluid of tadpole tissue. It cleaves the native molecule into two fragments in a highly specific fashion at a temperature below that of substrate denaturation. Since then, collagenolytic enzymes have been obtained from a wide range of animal tissues. In general, these enzymes have fundamental properties in common; they all have a neutral pH optima and are not stored within the cell, but rather are secreted either in an inactive form or bound to inhibitors.

They appear to be zinc metalloenzymes requiring calcium, and are not inhibited by agents that block serine or sulphydryl-type proteinases. Nearly all the collagenases studied so far have a molecular mass that ranges from 25,000 to 60,000 Daltons. Mammalian collagenases display a great deal of specificity, cleaving bands between Gly-Leu or Gly-Ile. There are slight differences in the amino acid sequences surrounding the scission site; these may account for the differences in the rates at which various collagens are degraded.

The enzymes interact tightly with the collagen fibers and appear to remain bound to the macromolecular aggregate during the degradation process. Approximately 10% of the collagen molecules in reconstituted collagen fibrils appear accessible for binding, in close agreement with the theoretical number of molecules estimated to be present near the surface of the fiber. The in vitro data obtained seem to indicate that digestion proceeds to completion by hopping from one molecule to another without returning to the solution. Collagen from older individuals is more resistant to enzymatic digestion.